This invention relates to telephone systems and, in particular, to transferring data within the voice band over a telephone line during a conversation. As used herein, a telephone “line” includes cellular telephones.
At present, there are two kinds of echo in a telephone system, an acoustic echo between an earphone or a speaker and a microphone and an electrical echo occurring in the switched network for routing a call between stations. In a handset, acoustic echo is typically not much of a problem. In speaker phones, where several people huddle around a microphone and loudspeaker, acoustic feedback is much more of a problem. Hybrid circuits (two-wire to four-wire transformers) located at terminal exchanges or in remote subscriber stages of a fixed network are the principal sources of electrical echo, also known as line echo.
An echo is perceived as an echo if the delay is greater than approximately fifty milliseconds. Acoustic echoes and line echoes typically far exceed this threshold. Between about twenty milliseconds and about fifty milliseconds, an echo can impart a certain richness to a sound, as is often done to enhance the thin voices of some recording artists.
It has been discovered that imperceptible echoes, that is, echoes having a delay less than about fifty milliseconds, can be used to transmit data in the voice band during a telephone conversation. The need for such capability has long existed. Telephones, and particularly cellular telephones, transmit considerable amounts of data prior to completing a call, i.e. prior to making a connection to the other party. Some data is transmitted after a party hangs up. The problem is that no data is transmitted during a call. The reason is obvious, no one wants a telephone beeping away in the background or the hiss of a multiplexed signal during a call.
There are many occasions when it would be desirable and extremely useful to be able to send data during a call. For example, in conference calls, one party invariably sounds louder than another and there is no way to make an adjustment during a call, except to interrupt the conversation to ask the louder party to speak more softly, which the louder party forgets to do after a few minutes.
There are many other situations that could be improved by being able to send data during a telephone call. For example, the time remaining on a pre-paid call could be sent and displayed at the payer's telephone. Data concerning routing or the quality of the line could also be exchanged.
Filtering a voice signal to eliminate echo is known in the art. Devices known as complementary comb filters have been used to eliminate echoes by having the signal to a speaker filtered through the pass bands of a first comb filter, thereby falling within the stop bands of a second, complementary comb filter coupled to a microphone. If all telephones are configured the same way, some sort of spectrum shift must take place to move undesired signals into the stop bands of a comb filter in order to eliminate both acoustic echoes and line echoes. U.S. Pat. No. 5,386,465 (Addeo et al.) discloses a frequency “scaler” for moving signals into a stop band.
The same type of comb filter can be used in each channel of a telephone if one also uses a frequency shift, see U.S. Pat. No. 4,748,663 (Phillips et al.). Frequency shifting and frequency scaling are undesirable because of their effect on the quality of the voice signal. Providing complementary filters would be simple and effective if telephones could communicate with each other during a call to select which filter to use to assure a complementary relation rather than both telephones using the same pass bands and stop bands.
In view of the foregoing, it is therefore an object of the invention to provide an apparatus and method for communicating data unobtrusively within the voice band over a telephone line during a conversation.
Another object of the invention is to communicate data, including control signals, over a telephone line during a conversation.
A further object of the invention is to provide an apparatus and method for communicating data over a telephone line simultaneously with voice signals, i.e. without multiplexing voice and data.
Another object of the invention is to provide an apparatus and method for controlling complementary comb filters during a telephone conversation.
A further object of the invention is to provide an apparatus and method for controlling a plurality of telephones in what is known as a conference call.